1. Field of the Invention
The present invention relates to an objective lens element used for performing at least one of recording, reproducing, or erasing of information on an information recoding surface of an optical information storage medium, and an optical pickup device including the objective lens element.
2. Description of the Background Art
In recent years, research and development has been actively carried out concerning high-density optical discs that have an increased recording density by using a blue laser beam with a wavelength of about 400 nm and thus have an improved storage capacity. One of the standards of such high-density optical discs is Blu-Ray Disc (registered trademark; hereinafter, referred to as “BD”) in which the image side numerical aperture (NA) of an objective lens is set to about 0.85 and the thickness of a protective base plate on an information recoding surface of an optical disc is set to about 0.1 mm.
Other than BD, DVD (protective base plate thickness: about 0.6 mm) for which a red laser beam with a wavelength of about 680 nm is used, and CD (protective base plate thickness: about 1.2 mm) for which an infrared laser beam with a wavelength of about 780 nm is used also exist. Various objective lenses that are compatible with three types of standards of these discs have been proposed.
For example, Japanese Patent No. 3993870 discloses an optical element and an optical pickup device that are compatible with the three types of the standards of BD, DVD, and CD. An objective lens disclosed in Japanese Patent No. 3993870 is provided with a stair-like diffraction structure (also referred to as binary type diffraction structure) in which stair-like steps are periodically arranged. The height of each step is set such that a difference in optical path of about 1.25 wavelengths is provided to a light beam having a shortest designed wavelength. In addition, one periodic structure consists of four steps that are consecutive in a radial direction (the height from a base surface is 0 to 3 times that of a unit step).
Since such a step structure is provided, the diffraction efficiency of a +1st order diffracted light beam can be at its maximum when a light beam of a wavelength for BD is used, and the diffraction efficiency of a −1st order diffracted light beam can be at its maximum when a light beam of a wavelength for DVD is used. Thus, by using change in an angle of diffraction with respect to a wavelength, it is possible to compensate for a spherical aberration that occurs due to differences in wavelength and disc base material thickness when changing between BD and DVD.
Further, Japanese Laid-Open Patent Publication No. 2005-243151 discloses an optical element that is compatible with a plurality of standards and that is provided with sawtooth-like diffraction structures having different depths.
FIG. 20 is a cross-sectional view of a principal part of conventional sawtooth-like diffraction structures disclosed in Japanese Laid-Open Patent Publication No. 2005-243151.
In Japanese Laid-Open Patent Publication No. 2005-243151, in order to increase the diffraction order (diffraction angle) in an outer region, a cycle (pitch) PB and a depth HB of a sawtooth-like diffraction structure provided in a region R22 on the outer side are set so as to be twice that of a cycle (pitch) PA and a depth HA of a sawtooth-like diffraction structure provided in a region R21 on the inner side.
As described in Japanese Laid-Open Patent Publication No. 2005-243151, in a configuration in which the shape of the diffraction structure is different from region to region, it is necessary that the phases of light beams passing outside and inside the boundary between adjacent regions be made to coincide, in order that a light beam diffracted at each region is converged on one spot.
However, in the configuration in Japanese Laid-Open Patent Publication No. 2005-243151, when a designed wavelength or a material refractive index changes, a phase mismatch occurs between a light beam diffracted by the region R21 and a light beam diffracted by the region R22.
Specifically, in the configuration in Japanese Laid-Open Patent Publication No. 2005-243151, the height and the cycle of the grating provided in the region R22 on the outer side are set so as to be twice that of the height and the cycle of the grating provided in the region R21 on the inner side, but the direction of the diffraction by the region R21 is the same as the direction of the diffraction by the region R22. In this case, the phase of a light beam having passed through a point C21 near the boundary between the regions R21 and R22 coincides with the phase of a light beam having passed through a point C22 near the boundary when a difference in optical path length caused by the grating height HA is an integral multiple of the wavelength, in other words, when a phase difference is an integral multiple of 2π.
However, in the case of mass-produced products such as optical disc devices, an inevitable variation of several nanometers occurs in the wavelength of a semiconductor laser used as a light source. In addition, due to a difference in temperature of operating environment, the wavelength also changes. Even when a use wavelength is deviated from a design center, the direction of diffraction depends on the relation between the grating pitch and the wavelength. Thus, the direction of diffraction by each region is the same, and a disagreement of the direction of diffraction does not occur between the regions, but phase matching is not kept.
Here, in FIG. 20, the case is assumed where the upper surface of the diffraction structure is an exit surface and light beams having the same phase are incident on the diffraction structure from the lower side in FIG. 20. On a base surface BA shown in FIG. 20, the phases of the light beams are the same. When the light beams travel across the base surface BA in the upward direction in the drawing in a glass material having a refractive index different from that of the air, a phase difference occurs between the phase of the light beams travelling in the grass material and the phase of light travelling in the air, due to the influence of the sawtooth-like diffraction grating. When light beams passing in a range of from the points C20 to C22 shown in FIG. 20 are considered, a light beam having passed through the point C20 is outputted to the air immediately after passing across the base surface BA, and thus no phase difference occurs. In the range of from the points C20 to C22, a light beam passing through the point C21 slightly inward of the point C22 travels in the glass material for the longest distance after passing across the base surface BA. Thus, the light beam having passed through the point C21 has a maximum phase difference from the phase in the case of traveling in the air. The phase difference changes in proportion to the wavelength, and the change amount of the phase difference of the light beam passing through the point C21 is at its maximum in the range of from the points C20 to C22. The phase change amount of a diffracted light beam can be represented by the average in the range of from the points C20 to C22, and the phase difference provided to an incident light beam by the diffraction structure in the region R21 is equal to that in the case where the glass material extends to a line M211 (the center line of the amplitudes of the sawteeth in the region R21) shown in FIG. 20. Similarly, the phase difference provided to an incident light beam by the diffraction structure in the region R22 on the outer side is equal to that in the case where the glass material extends to a line M212 (the center line of the amplitudes of the sawteeth in the region R22) shown in FIG. 20.
Since a phase difference between outputted light beams that occurs when it is assumed that there is an average exit surface along the line M211 in FIG. 20 is not the same as a phase difference between outputted light beams that occurs when it is assumed that an average exit surface along the line M212, a phase change amount provided when the wavelength of an incident light beam changes is different between the regions R21 and R22. Thus, when the wavelength of an incident light beam changes, the diffraction direction of a light beam diffracted by the region R21 is the same as the diffraction direction of a light beam diffracted by the region R22, but a phase shift occurs between these light beams, leading to a decrease in diffraction efficiency. Further, when a light beam having passed through the region R21 and a light beam having passed through the region R22 are converged, an aberration occurs.
As described above, in the conventional art, objective lenses in which diffraction structures having different shapes are provided in regions, respectively, have been proposed. However, a phase shift caused by an error of the wavelength of a light source when a light beam diffracted by each region is converged is not taken into consideration in such designs.